This invention relates to sheet stacking devices and, particularly, to a device for stacking sheets delivered seriatim to the device at the exit end of a printing machine or the like.
Printing, copying or duplicating machines, such as rotary offset lithographic duplicating machines, normally are provided with some form of sheet receiving means at the exit end of the machine for stacking copy sheets issuing from the machine. Conventionally, the sheet receiving means comprises a receiving tray for receiving and stacking the sheets as the sheets fall by gravity and come to rest onto the top of a stack in the tray. Periodically, or when the tray is full, the stack of sheets is removed from the tray by the machine operator. Prior devices of this general kind have been fairly successful in instances where the duplicating machine speed is maintained at approximately 8,000 or less impressions per hour (IPH). However, such prior devices cannot handle stacking sheets at higher machine speeds such as on the order of 9,000 to 20,000 IPH. At the higher speeds, the sheets cannot be controlled and therefore jamming results.
There are various causes of sheet jamming at high machine speeds. First, prior machines normally do not have adequate means for directing the sheets outwardly over the tray. The lead ends of the sheets issuing from the machine drop too quickly before the sheets are directed a sufficient distance outwardly over the tray and therefore the sheets tend to roll or curl as the sheets fall into the tray. Attempts have been made to solve this problem by including flexible straps or deflectors acting against a substantial portion of each sheet to urge the sheets downwardly into the tray in an attempt to control the sheets being deposited in the tray. However, such flexible straps create considerable static electricity at high speeds and cause the sheets to adhere to the straps thereby resulting in sheet jamming.
Another problem with handling sheets at high machine speeds is in controlling upward deflection of the lead ends of the sheets. As the sheets issue from the machine, there often is a tendency for the lead ends of the sheets to "hang" or flip upwardly due to air resistance and further causes jamming.
Probably the most prevalent cause of sheet jamming at high machine speeds simply resides in the inability of a sheet to fall sufficiently fast to clear a path for the immediately succeeding sheet. Heretofore there has been no adequate means for controlling the trail ends of the sheets to avoid sheet jamming at high speeds. Because there is only a small gap between the trail end of a given sheet and the lead end of a succeeding sheet issuing from the machine, means must be provided for controlling the trail ends of the sheets to cause the trail end of a given sheet to drop immediately and clear a path for the lead end of an oncoming sheet in order to handle sheets issuing from the machine at high speed.
The invention is directed to a device for facilitating stacking the sheets into the tray at high machine speeds and avoiding the aforementioned problems of sheet jamming.